Unexpected Influence of the Neutral Lipid Nature on pH-Regulated Release of Salt from the Anionic Fliposomes
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nexpected Influence of the Neutral Lipid Nature on pH-Regulated Release of Salt from the Anionic Fliposomes K. M. Imelbaevaa and D. A. Stepanovaa,* a Lomonosov
Moscow State University, Moscow, 119991 Russia *e-mail: [email protected]
Received February 11, 2020; revised April 4, 2020; accepted April 14, 2020
Abstract—It was found that the substitution of electoneutral dioleoylphosphatidylcholine with dipalmytoylphosphatidylcholine in membrane of anionic fliposomes with molar fraction of pH-sensitive 4,5-bis-dodecylcarbonyl2-morpholincyclohexanol 0.3 does not result in formation of expected gel-state liposomes. Nevertheless, this change in composition of lipid membrane strongly affects on kinetics of pH-governed release of incapsulated in fliposomes salt. Keywords: fliposomes, pH-sensivity, lipid membrane, conformational switch, controlled release
DOI: 10.1134/S107036322004026X to shift of protonation-deprotonation equilibrium as it presented on Fig. 1. The driving force of this perturbation is intramolecular hydrogen bond of HO···H–N+ type and an electrostatic/ dipole-dipole attraction stabilizing a conformer with the gauche form of O–C–C–N fragment (Fig. 1). This later results in a conformational flip of the cyclohexane ring that moves the ester groups COOR at the other end of the molecule away from each other into axial positions. The relocation of substituents of molecules incorporated in lipid bilayer changes their intra- and intermolecular interactions disordering package of the bilayer. The role of flipid on efficiency of pH-responsive release of low-molecular weight substances was discussed in number of publications. It was shown that decrease of flipid molar ratio in membrane results in dramatical decrease of the fraction of released content from liposomal inner cavity [1, 10]. On the other hand authors of [10, 11] have demonstrated that modification of anionic fliposomes with polycation leads to drastically increase of the release efficiency. The rate of the release and the share of the released content for the complexes were higher than for the individual fliposomes. Nevertheless, the 100% efficiency of the release was not demonstrated for the majority of the fliposomes compositions. It seems that lateral diffusion of the membrane lipids leads to “healing” of the defects caused by flipid rearrangement after protonation and as a result stops the release process. It is reasonable to assume that manipulation with lipid
INTRODUCTION Fliposomes are the stimuli-responsive unilamelar bilayred vesicles containing in their membrane lipidmimetic surfactant capable to conformational change in response to changes in surrounding media [1]. In turn this conformational rearrangement-flip-disorders lipid membranes of vesicles [2]. This specific class of lipid-like surfactants was named flipids [1]. Incorporation of flipids in membrane of liposomes allows one to manipulate with their integrity thus helping in formation of nanocontainers with controlled decapsulation. Hydrophilic substances incorporated in inner
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